[csharp][unity] Fixed compile errors on older Unity/C# versions.

spine-csharp/src/Animation.cs

@@ -54,7 +54,7 @@ namespace Spine {
 			this.name = name;
 			this.duration = duration;
 			int n = timelines.Count << 1;
-			timelineIds = new HashSet<string>(n);
+			// note: not needed: timelineIds = new HashSet<string>(n);
 			bones = new ExposedList<int>(n);
 			SetTimelines(timelines);
 		}
@@ -2106,7 +2106,7 @@ namespace Spine {
 		/// <summary>
 		/// The index of the constraint in <see cref="Skeleton.Constraints"/> that will be changed when this timeline is applied.
 		/// </summary>
-		public int ConstraintIndex { get; }
+		int ConstraintIndex { get; }
 	}
 
 	/// <summary>Changes an IK constraint's <see cref="IkConstraintPose.Mix"/>, <see cref="IkConstraintPose.Softness"/>,

spine-csharp/src/Attachments/IHasTextureRegion.cs

@@ -51,9 +51,9 @@ namespace Spine {
 		/// </summary>
 		void UpdateRegion ();
 
-		public abstract Color GetColor ();
+		Color GetColor ();
-		public abstract void SetColor (Color color);
+		void SetColor (Color color);
-		public abstract void SetColor (float r, float g, float b, float a);
+		void SetColor (float r, float g, float b, float a);
 
 		Sequence Sequence { get; set; }
 	}

spine-csharp/src/ConstraintData.cs

@@ -33,8 +33,8 @@ using System.Collections.Generic;
 namespace Spine {
 
 	public interface IConstraintData : IPosedData {
-		public string Name { get; }
+		string Name { get; }
-		public IConstraint Create (Skeleton skeleton);
+		IConstraint Create (Skeleton skeleton);
 	}
 
 	public abstract class ConstraintData<T, P> : PosedData<P>, IConstraintData

spine-csharp/src/IPose.cs

@@ -31,6 +31,6 @@ using System;
 
 namespace Spine {
 	public interface IPose<P> {
-		public void Set (P pose);
+		void Set (P pose);
 	}
 }

spine-csharp/src/PosedData.cs

@@ -31,7 +31,7 @@ using System;
 
 namespace Spine {
 	public interface IPosedData {
-		public bool SkinRequired { get; }
+		bool SkinRequired { get; }
 	}
 
 	/// <summary>